/* double exponentiation r=x^e.y^f mod p */
public void pow2(BIG e, FF y, BIG f, FF p)
{
int i, eb, fb, n = p.length;
FF xn = new FF(n);
FF yn = new FF(n);
FF xy = new FF(n);
FF ND = p.invmod2m();
xn.copy(this);
yn.copy(y);
xn.nres(p);
yn.nres(p);
xy.copy(xn);
xy.modmul(yn, p, ND);
one();
nres(p);
for (i = 8 * ROM.MODBYTES - 1; i >= 0; i--)
{
eb = e.bit(i);
fb = f.bit(i);
modsqr(p, ND);
if (eb == 1)
{
if (fb == 1)
{
modmul(xy, p, ND);
}
else
{
modmul(xn, p, ND);
}
}
else
{
if (fb == 1)
{
modmul(yn, p, ND);
}
}
}
redc(p, ND);
}